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Head direction cells
Head direction cells
Many mammals possess neurons called head direction (HD) cells, which are active only when the animal's head
points in a specific direction within an environment. These neurons fire at a steady rate (i.e. they do not show
adaptation), but show a decrease in firing rate down to a low baseline rate as the animal's head turns away from the
preferred direction (usually returning to baseline when facing about 45° away from this direction).
These cells are found in many brain areas, including the post-subiculum, retrosplenial cortex, the thalamus (the
anterior and the lateral dorsal thalamic nuclei), lateral mammillary nucleus, dorsal tegmental nucleus, striatum and
entorhinal cortex (Sargolini et al, Science, 2006).
The system is related to the place cell system, which is mostly orientation-invariant and location-specific, while HD
cells are mostly orientation-specific and location-invariant. However, HD cells do not require a functional
hippocampus, where strong place cells are found, to show their head direction specificity. Head direction cells are
not sensitive to geomagnetic fields (i.e. they are not "magnetic compass" cells), and are neither purely driven by nor
are independent of sensory input. They strongly depend on the vestibular system, and the firing is independent of the
position of the animal's body relative to its head.
Some HD cells exhibit anticipatory behaviour: the best match between HD activity and the animal's actual head
direction has been found to be up to 95 ms in future. That is, activity of head direction cells predicts, 95 ms in
advance, what the animal's head direction will be.
Vestibular influences
The HD compass is inertial: it continues to operate even in the absence of light. Experiments have shown that the
inertial properties are dependent on the vestibular system, especially the semicircular canals of the inner ear, which
respond to rotations of the head. The HD system integrates the vestibular output to maintain a signal of cumulative
rotation. The integration is less than perfect, though, especially for slow head rotations. If an animal is placed on an
isolated platform and slowly rotated in the dark, the alignment of the HD system usually shifts a little bit for each
rotation. If an animal explores, in the dark, an environment with no directional cues, the HD alignment tends to drift
slowly and randomly over time.
Visual influences
One of the most interesting aspects of head direction cells is that their firing is not fully determined by sensory
features of the environment. When an animal comes into a novel environment for the first time, the alignment of the
head direction system is arbitrary. Over the first few minutes of exploration, the animal learns to associate the
landmarks in the environment with directions. When the animal comes back into the same environment at a later
time, if the head direction system is misaligned, the learned associations serve to realign it.
It is possible to temporarily disrupt the alignment of the HD system, for example by turning out the lights for a few
minutes. Even in the dark, the HD system continues to operate, but its alignment to the environment may gradually
drift. When the lights are turned back on and the animal can once more see landmarks, the HD system usually comes
rapidly back into the normal alignment. Occasionally the realignment is delayed: the HD cells may maintain an
abnormal alignment for as long as a few minutes, but then abruptly snap back.
If these sorts of misalignment experiments are done too often, the system may break down. If an animal is repeatedly
disoriented, and then placed into an environment for a few minutes each time, the landmarks gradually lose their
ability to control the HD system, and eventually, the system goes into a state where it shows a different, and random,
alignment on each trial.
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Head direction cells
There is evidence that the visual control of HD cells is mediated by the postsubiculum. Lesions of the postsubiculum
do not eliminate thalamic HD cells, but they often cause the directionality to drift over time, even when there are
plenty of visual cues. Thus, HD cells in postsub-lesioned animals behave like HD cells in intact animals in the
absence of light. Also, only a minority of cells recorded in the postsubiculum are HD cells, and many of the others
show visual responses.In familiar environments, HD cells show consistent preferred directions across time as long as
there is a polarizing cue of some sort that allows directions to be identified (in a cylinder with unmarked walls and
no cues in the distance, preferred directions may drift over time).
History
Head direction cells were first noticed by James B. Ranck, Jr., in the rat postsubiculum, a structure that lies near the
hippocampus on the dorsocaudal brain surface. Ranck reported his discovery in a brief abstract in 1984. A
postdoctoral fellow working in his laboratory, Jeffrey S. Taube, made these cells the subject of his research, and
summarized his findings in a pair of papers in the Journal of Neuroscience in 1990.[1] [2] These seminal papers
served as the foundation for all of the work that has been done subsequently. Taube, after taking a position at
Dartmouth College, has devoted his career to the study of head direction cells, and been responsible for a number of
the most important discoveries, as well as writing several key review papers.
The postsubiculum has numerous anatomical connections. Tracing these connections led to the discovery of head
direction cells in other parts of the brain. In 1993, Mizumori and Williams reported finding HD cells in a small
region of the rat thalamus called the lateral dorsal nucleus.[3] Two years later, Taube found HD cells in the nearby
anterior thalamic nuclei.[4] Chen et al. found limited numbers of HD cells in posterior parts of the neocortex.[5] The
observation in 1998 of HD cells in the lateral mammillary area of the hypothalamus completed an interesting pattern:
the parahippocampus, mammillary nuclei, anterior thalamus, and retrosplenial cortex are all elements in a neural
loop called the Papez circuit, proposed by Walter Papez in 1939 as the neural substrate of emotion. Limited numbers
of robust HD cells have also been observed in the hippocampus and dorsal striatum. Recently, substantial numbers of
HD cells have been found in the medial entorhinal cortex, intermingled with spatially-tuned grid cells.
The remarkable properties of HD cells, most particularly their conceptual simplicity and their ability to maintain
firing when visual cues were removed or perturbed, led to considerable interest from theoretical neuroscientists.
Several mathematical models were developed, which differed on details but had in common a dependence on
mutually excitatory feedback to sustain activity patterns: a type of working memory, as it were.[6]
For a review on the HD system and place field system, see Muller (1996): “A quarter of a Century of Place Cells”,
Sharp et al. (2001): “The anatomical and computational basis of rat HD signal”.
Notes
[1]
[2]
[3]
[4]
[5]
[6]
Taube et al., 1990a
Taube et al., 1990b
Mizumori and Williams, 1993
Taube, 1995
Chen et al., 1994
Zhang, 1996
References
• Blair, HT; Cho J, Sharp PE (1998). "Role of the lateral mammillary nucleus in the rat head direction circuit: a
combined single unit recording and lesion study.". Neuron 21 (6): 1387–1397.
doi:10.1016/S0896-6273(00)80657-1. PMID 9883731.
• Chen, LL; Lin LH, Green EJ, Barnes CA, McNaughton BL (1994). "Head-direction cells in the rat posterior
cortex. I. Anatomical distribution and behavioral modulation.". Exp. Brain Res. 101 (1): 8–23.
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Head direction cells
doi:10.1007/BF00243212. PMID 7843305.
• Mizumori, SJ; Williams JD (September 1, 1993). "Directionally selective mnemonic properties of neurons in the
lateral dorsal nucleus of the thalamus of rats." (http://www.jneurosci.org/cgi/content/abstract/13/9/4015). J.
Neurosci. 13 (9): 4015–4028. PMID 8366357.
• Taube, JS; Muller RU, Ranck JB Jr. (1 February 1990). "Head-direction cells recorded from the postsubiculum in
freely moving rats. I. Description and quantitative analysis." (http://www.jneurosci.org/cgi/content/abstract/
10/2/420). J. Neurosci. 10 (2): 420–435. PMID 2303851.
• Taube, JS; Muller RU, Ranck JB Jr. (1990b). "Head-direction cells recorded from the postsubiculum in freely
moving rats. II. Effects of environmental manipulations." (http://www.jneurosci.org/cgi/content/abstract/10/
2/436). J. Neurosci. 10 (2): 436–447. PMID 2303852.
• Taube, JS (January 1, 1995). "Head direction cells recorded in the anterior thalamic nuclei of freely moving rats."
(http://www.jneurosci.org/cgi/content/abstract/15/1/70). J. Neurosci. 15 (1): 70–86. PMID 7823153.
• Taube, JS (2007). "The head direction signal: Origins and sensory-motor integration." (http://arjournals.
annualreviews.org/doi/abs/10.1146/annurev.neuro.29.051605.112854). Ann. Rev. Neurosci. 30: 181–207.
doi:10.1146/annurev.neuro.29.051605.112854. PMID 17341158.
• Zhang, K (March 15, 1996). "Representation of spatial orientation by the intrinsic dynamics of the head-direction
cell ensemble: a theory." (http://www.jneurosci.org/cgi/content/abstract/16/6/2112). J. Neurosci. 16 (6):
2112–2126. PMID 8604055.
3
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4
Grid cell
1
Grid cell
A grid cell is a type of neuron that has been found in the brains of
rats and mice; and it is likely to exist in other animals including
humans.[1] [2] [3] . In a typical experimental study, an electrode
capable of recording the activity of an individual neuron is
implanted in the cerebral cortex of a rat, in a part called the
dorsomedial entorhinal cortex, and recordings are made as the rat
moves around freely in an open arena. For a grid cell, if a dot is
placed at the location of the rat's head every time the neuron emits
an action potential, then as illustrated in the adjoining figure, these
dots build up over time to form a set of small clusters, and the
clusters form the vertices of a grid of equilateral triangles. This
regular triangle-pattern is what distinguishes grid cells from other
types of cells that show spatial firing correlates. By contrast, if a
Trajectory of a rat through a square environment is
place cell from the rat hippocampus is examined in the same way
shown in black. Red dots indicate locations at which a
particular entorhinal grid cell fired.
(i.e., by placing a dot at the location of the rat's head whenever the
cell emits an action potential), then the dots build up to form small
clusters, but frequently there is only one cluster (one "place field") in a given environment, and even when multiple
clusters are seen, there is no perceptible regularity in their arrangement.
Grid cells were discovered in 2005 by Edvard Moser, May-Britt Moser and
their students Torkel Hafting, Marianne Fyhn and Sturla Molden at the then
Centre for the Biology of Memory (CBM) in Norway. The arrangement of
spatial firing fields all at equal distances from their neighbors led to a
hypothesis that these cells encode a cognitive representation of Euclidean
space.[1] The discovery also suggested a mechanism for dynamic computation
of self-position based on continuously updated information about position and
direction.
What makes grid cells especially interesting is that the regularity in grid
spacing does not derive from any regularity in the environment or in the
Grid cells derive their name from the fact
that connecting the centers of their firing
sensory input available to an animal. In other words, grid cells appear to
fields gives a triangular grid.
encode a type of abstract spatial structure that is constructed inside the brain
and imposed on the environment by the brain with no regard for the sensory
features of the environment. Thus, the discovery of grid cells may provide a verification of Immanuel Kant's theory
that Euclidean space constitutes a synthetic a priori—a structure that is not purely logical but is constructed by the
mind without requiring information from the environment. (In more modern terminology, a "synthetic a priori" is a
structure that comes from nature rather than nurture; i.e., a structure that is innate rather than learned).[4]
Background
In 1971, John O'Keefe and Jonathon Dostrovsky reported the discovery of place cells in the rat hippocampus—cells
that fire action potentials when an animal passes through a specific small region of space, which is called the place
field of the cell.[5] This discovery, although controversial at first, led to a series of investigations that culminated in
the 1978 publication of a book by O'Keefe and his colleague Lynn Nadel called The Hippocampus as a Cognitive
Map[6] —the book argued that the hippocampal neural network instantiates cognitive maps as hypothesized by the
psychologist Edward C. Tolman. This theory aroused a great deal of interest, and motivated hundreds of
Grid cell
2
experimental studies aimed at clarifying the role of the hippocampus in spatial memory and spatial navigation.
Because the entorhinal cortex provides by far the largest input to the hippocampus, it was clearly important to
understand the spatial firing properties of entorhinal neurons. The earliest studies, such as Quirk et al. (1992),
described neurons in the entorhinal cortex as having relatively large and fuzzy place fields.[7] The Mosers, however,
thought it was possible that a different result would be obtained if recordings were made from a different part of the
entorhinal cortex. The entorhinal cortex is a strip of tissue running along the back edge of the rat brain from the
ventral to the dorsal sides. Anatomical studies had shown that different sectors of the entorhinal cortex project to
different levels of the hippocampus: the dorsal end of the EC projects to the dorsal hippocampus, the ventral end to
the ventral hippocampus.[8] . This was relevant because several studies had shown that place cells in the dorsal
hippocampus have considerably sharper place fields than cells from more ventral levels.[9] Every study of entorhinal
spatial activity prior to 2004, however, had made use of electrodes implanted near the ventral end of the EC.
Accordingly, the Marianne Fyhn in the Moser group set out to examine spatial firing from the different modules of
entorhinal cortex. The first results were reported in Fyhn et al. (2004), which described cells from the dorsomedial
EC with sharply defined place fields at multiple locations.[10] In these initial data, the arrangement of fields for many
cells showed hints of regularity, but the size of the environment was too small for large numbers of fields to appear,
so no firm conclusions were drawn. The next set of experiments, reported in 2005, made use of a larger environment
where the grid pattern was strikingly easy to observe.[1] .
Properties
Grid cells are neurons that fire when a freely moving animal
traverses a set of small regions (firing fields) which are roughly
equal in size and arranged in a periodic triangular array that covers
the entire available environment.[1] Cells with this firing pattern
have been found in all layers of the dorsocaudal medial entorhinal
cortex (dMEC), but cells in different layers tend to differ in other
respects. Layer II contains the largest density of pure grid cells, in
the sense that they fire equally regardless of the direction in which
an animal traverses a grid location. Grid cells from deeper layers
are intermingled with cells with conjunctive grid and head
direction properties(i.e. in layers III, V and VI there are cells with
a grid-like pattern that fire only when the animal is facing a
particular direction).[11]
Spatial autocorrelogram of the neuronal activity of the
grid cell from the first figure.
Grid cells that lie next to one another (i.e., cells recorded from the same electrode) usually show the same grid
spacing and orientation, but their grid vertices are displaced from one another by apparently random offsets. Cells
recorded from separate electrodes at a distance from one another, however, frequently show different grid spacings.
Cells that are located more ventrally (that is, farther from the border between the entorhinal cortex and postrhinal
cortex) generally have larger firing fields at each grid vertex, and correspondingly greater spacing between the grid
vertices.[1] The total range of grid spacings is not well established: the initial report described a roughly twofold
range of grid spacings (from 39 cm to 73 cm) across the dorsalmost part (upper 25%) of the MEC[1] , but there are
indications of considerably larger grid scales in more ventral zones. Brun et al. (2008) recorded grid cells from
multiple levels in rats running along an 18 meter track, and found that the grid spacing expanded from about 25 cm
in their dorsalmost sites to about 3 m at the ventralmost sites.[12] These recordings only extended 3/4 of the way to
the ventral tip, so it is possible that even larger grids exist.
Grid cell activity does not require visual input, since grid patterns remain unchanged when all the lights in an
environment are turned off.[1] When visual cues are present, however, they exert strong control over the alignment of
Grid cell
the grids: rotating a cue card on the wall of a cylinder causes grid patterns to rotate by the same amount.[1] Grid
patterns appear on the first entrance of an animal into a novel environment, and usually remain stable thereafter.[1]
When an animal is moved into a completely different environment, grid cells maintain their grid spacing, and the
grids of neighboring cells maintain their relative grid vertex offsets.[1]
Interactions with hippocampal place cells
When a rat is moved to a different environment, the spatial activity patterns of hippocampal place cells usually show
"complete remapping"—that is, the pattern of place fields reorganizes in a way that bears no detectable resemblance
to the pattern in the original environment. If the features of an environment are altered less radically, however, the
place field pattern may show a lesser degree of change, referred to as "rate remapping", in which many cells alter
their firing rates but the majority of cells retain place fields in the same locations as before. Fyhn et al. (2007)
examined this phenomenon using simultaneous recordings of hippocampal and entorhinal cells, and found that in
situations where the hippocampus shows rate remapping, grid cells show unaltered firing patterns, whereas when the
hippocampus shows complete remapping, grid cell firing patterns show unpredictable shifts and rotations.[13]
Theta rhythmicity
Neural activity in nearly every part of the hippocampal system is modulated by the limbic theta rhythm, which has a
frequency range of about 6–9 Hz in rats. The entorhinal cortex is no exception: like the hippocampus, it receives
cholinergic and GABAergic input from the medial septal area, the central controller of theta. Grid cells, like
hippocampal place cells, show strong theta modulation[1] . Grid cells from layer II of the MEC also resemble
hippocampal place cells in that they show phase precession—that is, their spike activity advances from late to early
phases of the theta cycle as an animal passes through a grid vertex. Most grid cells from layer III do not precess, but
their spike activity is largely confined to half of the theta cycle. The grid cell phase precession is not derived from
the hippocampus, because it continues to appear in animals whose hippocampus has been inactivated by a local
anesthetic.[14]
Possible functions
Many species of mammals can keep track of spatial location even in the absence of visual, auditory, olfactory, or
tactile cues, by integrating their movements—the ability to do this is referred to in the literature as path integration.
A number of theoretical models have explored mechanisms by which path integration could be performed by neural
networks. In most models, such as those of Samsonovich and McNaughton (1997)[15] or Burak and Fiete (2009),[16]
the principal ingredients are (1) an internal representation of position, (2) internal representations of the speed and
direction of movement, and (3) a mechanism for shifting the encoded position by the right amount when the animal
moves. Because cells in the MEC encode information about position (grid cells[1] ) and movement (head direction
cells and conjunctive position-by-direction cells[11] ), this area is currently viewed as the most promising candidate
for the place in the brain where path integration occurs. However, the question remains unresolved, as in humans the
entorhinal cortex does not appear to be required for path integration[17] . Burak and Fiete (2009) showed that a
computational simulation of the grid cell system was capable of performing path integration to a high level of
accuracy.[16]
Hafting et al. (2005) suggested that a place code is computed in the entorhinal cortex and fed into the hippocampus,
which may make the associations between place and events that are needed for the formation of memories.
In contrast to a hippocampal place cell, a grid cell has multiple firing fields, with regular spacing, which tessellate
the environment in a hexagonal pattern. The unique properties of grid cells are as follows:
1. Grid cells have firing fields dispersed over the entire environment (in contrast to place fields which are restricted
to certain specific regions of the environment)
2. The firing fields are organized into a hexagonal grid
3
Grid cell
3. Firing fields are generally equally spaced apart, such that the distance from one firing field to all six adjacent
firing fields is approximately the same (though when an environment is resized, the field spacing may shrink or
expand different in different directions; Barry et al. 2007)
4. Firing fields are equally positioned, such that the six neighboring fields are located at approximately 60 degree
increments
The grid cells are anchored to external landmarks, but persist in darkness, suggesting that grid cells may be part of a
self-motion based map of the spatial environment.
See also
• Border cells, discovered in 2008.
References
[1] Hafting, T.; Fyhn, M.; Molden, S.; Moser, M. -B.; Moser, E. I. (2005). "Microstructure of a spatial map in the entorhinal cortex". Nature 436
(7052): 801. doi:10.1038/nature03721. PMID 15965463.
[2] Fyhn, M.; Hafting, T.; Witter, M. P.; Moser, E. I.; Moser, M. B. (2008). "Grid cells in mice". Hippocampus 18 (12): 1230.
doi:10.1002/hipo.20472. PMID 18683845.
[3] Doeller, C. F.; Barry, C.; Burgess, N. (2010). "Evidence for grid cells in a human memory network". Nature 463 (7281): 657.
doi:10.1038/nature08704. PMID 20090680.
[4] Moser EI, Moser MB (2008). "A metric for space". Hippocampus 18: 1142–56. doi:10.1002/hipo.20483. PMID 19021254.
[5] O'Keefe J, Dostrovsky JO (1971). "The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat".
Brain Research 34 (1): 171–5. doi:10.1016/0006-8993(71)90358-1. PMID 5124915.
[6] O'Keefe J, Nadel L (1978). The Hippocampus as a Cognitive Map (http:/ / www. cognitivemap. net/ HCMpdf/ HCMChapters. html). Oxford
University Press. . Retrieved 2009-11-05.
[7] Quirk G, Muller RU, Kubie JL, Ranck JB Jr. (1992). "The positional firing properties of medial entorhinal neurons: description and
comparison with hippocampal place cells". Journal of Neuroscience 12 (5): 1945–63. PMID 1578279.
[8] Moser MB, Moser EI (1998). "Functional differentiation in the hippocampus". Hippocampus 8 (6): 608–19. PMID 9882018.
[9] Maurer, A. P.; Vanrhoads, S. R.; Sutherland, G. R.; Lipa, P.; McNaughton, B. L. (2005). "Self-motion and the origin of differential spatial
scaling along the septo-temporal axis of the hippocampus". Hippocampus 15 (7): 841–852. doi:10.1002/hipo.20114. PMID 16145692.
[10] Fyhn, M.; Molden, S.; Witter, M. P.; Moser, E. I.; Moser, M. -B. (2004). "Spatial Representation in the Entorhinal Cortex". Science 305
(5688): 1258. doi:10.1126/science.1099901. PMID 15333832.
[11] Sargolini, F.; Fyhn, M.; Hafting, T.; McNaughton, B. L.; Witter, M. P.; Moser, M. -B.; Moser, E. I. (2006). "Conjunctive Representation of
Position, Direction, and Velocity in Entorhinal Cortex". Science 312 (5774): 758. doi:10.1126/science.1125572. PMID 16675704.
[12] Brun, V. H.; Solstad, T.; Kjelstrup, K. B.; Fyhn, M.; Witter, M. P.; Moser, E. I.; Moser, M. B. (2008). "Progressive increase in grid scale
from dorsal to ventral medial entorhinal cortex". Hippocampus 18 (12): 1200. doi:10.1002/hipo.20504. PMID 19021257.
[13] Fyhn, M.; Hafting, T.; Treves, A.; Moser, M. B.; Moser, E. I. (2007). "Hippocampal remapping and grid realignment in entorhinal cortex".
Nature 446 (7132): 190. doi:10.1038/nature05601. PMID 17322902.
[14] Hafting, T.; Fyhn, M.; Bonnevie, T.; Moser, M. B.; Moser, E. I. (2008). "Hippocampus-independent phase precession in entorhinal grid
cells". Nature 453 (7199): 1248. doi:10.1038/nature06957. PMID 18480753.
[15] Samsonovich A, McNaughton BL (1997). "Path integration and cognitive mapping in a continuous attractor neural network model" (http:/ /
www. jneurosci. org/ cgi/ content/ full/ 17/ 15/ 5900). Journal of Neuroscience 17 (15): 5900–20. PMID 9221787. .
[16] Burak, Y.; Fiete, I. R.; Sporns, O. (2009). "Accurate Path Integration in Continuous Attractor Network Models of Grid Cells" (http:/ / www.
pubmedcentral. nih. gov/ articlerender. fcgi?tool=pmcentrez& artid=2632741). PLoS Computational Biology 5 (2): e1000291.
doi:10.1371/journal.pcbi.1000291. PMID 19229307. PMC 2632741.
[17] Shrager, Y.; Kirwan, C. B.; Squire, L. R. (2008). "Neural basis of the cognitive map: Path integration does not require hippocampus or
entorhinal cortex" (http:/ / www. pubmedcentral. nih. gov/ articlerender. fcgi?tool=pmcentrez& artid=2575247). Proceedings of the National
Academy of Sciences 105 (33): 12034. doi:10.1073/pnas.0805414105. PMID 18687893. PMC 2575247.
• Solstad, T., Boccara, C.N., Kropff, E., Moser, M.-B. and Moser, E.I. (2008). Representation of geometric borders
in the entorhinal cortex. Science, 322, 1865-1868 (http://www.sciencemag.org/cgi/content/abstract/322/
5909/1865).
• Moser EI & Moser MB. A metric for space. Hippocampus. 18(12):1142-56. (2008). PMID 19021254
4
Grid cell
External links
• Centre for the Biology of Memory (CBM) (http://www.cbm.ntnu.no/english)
5
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6
Place cell
1
Place cell
Place cells are neurons in the hippocampus
that exhibit a high rate of firing whenever an
animal is in a specific location in an
environment corresponding to the cell's
"place field". These neurons are distinct
from other neurons with spatial firing
properties, such as grid cells, border cells,
head direction cells, and spatial view cells.
In the CA1 and CA3 hippocampal subfields,
place cells are believed to be pyramidal
cells, while those in the dentate gyrus are
believed to be granule cells.[2]
Place cells were first described in rats by
O'Keefe and Dostrovsky.[3] Based on this
discovery, O'Keefe and Nadel hypothesized
that the primary function of the rat
hippocampus is to form a cognitive map of
the rat's environment.[4] Ekstrom and
colleagues have found cells with similar
properties in the human hippocampus, using
extracellular recordings from epilepsy
patients undergoing invasive monitoring of
their brain activity. [5]
Spatial firing patterns of 7 place cells recorded from the CA1 layer of a rat.The rat
ran several hundred laps clockwise around an elevated triangular track, stopping in
the middle of each arm to eat a small portion of food. Black dots indicate positions
of the rat's head; colored dots indicate action potentials, using a different color for
[1]
each cell.
Place fields
Place cells show increased frequency of firing when an animal is in a specific area referred to as the cell's place field.
The firing rate increase can be quite dramatic, from virtually zero outside the field to as much as 100 Hz (for brief
periods) in the middle of the place field. When a rat forages randomly in an environment, place fields are only
weakly modulated by the direction the rat faces, or not at all. However, when an animal engages in stereotyped
behaviour (e.g. shuttling between goal locations), place cells tend to be active in the place field on passes in one
direction only[6] .
On initial exposure to a new environment, place fields become established within minutes. The place fields of cells
tend to be stable over repeated exposures to the same environment. In a different environment, however, a cell may
have a completely different place field or no place field at all. This phenomenon is referred to as "remapping". In any
particular environment, roughly 40-50% of the hippocampal place cells will be active.[7] [8]
In an environment with few or no directional cues (for instance, a circular environment surrounded by black
curtains), place fields will tend to have a fixed radial position, but the entire set of place fields may rotate around the
maze as predicted by a theory that rats are slowly losing their orientation.[9] If a polarizing cue is introduced
(commonly a large white rectangle of paper), place fields will tend to have fixed positions relative to the cue. If the
cue is moved while the animal can see it, place fields will tend to remain unaffected; however, if the animal is briefly
removed from the environment then the cue is moved and the animal returned, the place fields will rotate so as to
maintain their position relative to the cue card. Although visual cues seem to be the primary determinant of place cell
Place cell
firing, it is worth noting that firing persists in the dark, suggesting that proprioception or other senses contribute as
well.
In an environment in which a rat is constrained to walk along a linear track, place fields will often have a directional
component in addition to a place component. A place cell that fires at a particular location while the rat walks in one
direction along the track will not necessarily fire as the rat visits that location from the other direction. If the rat
frequently turns around at the same point, however, place fields there will often be independent of direction.
The size of place fields and their signal to noise ratio varies depending on the region of brain in consideration. In the
hippocampus, place fields are smallest and sharpest at the dorsal pole, becoming larger toward the ventral pole.[10]
This may reflect the topography of projections to the hippocampus. For example, the ventral hippocampus receives
much more input from the amygdala, while dorsal hippocampus is more preferentially innervated by entorhinal
cortex.
Spatial modulated cells are also found in the entorhinal cortex, which feed input from neocortex into the
hippocampus. Neurons in the lateral entorhinal cortex exhibit little spatial selectivity,[11] while neurons of the medial
entorhinal (MEA) cortex exhibit multiple "place fields" that are arranged in an hexagonal pattern, and are therefore
called "grid cells". These fields and spacing between fields increase from the dorso-lateral MEA to the ventro-medial
MEA[12] [13]
2
Place cell
3
Phase precession
The hippocampus is one of many brain structures that can show a
characteristic 4-12 Hz oscillation, theta rhythm, in an EEG recording.
The oscillation has been observed in all mammalian species tested. In
both rats and humans, it is associated with real or virtual movement
through space.
When a neuron discharges, it can be said to fire in relation to the
current phase of a theta cycle (0-360 degrees). When a rat enters a
cell's place field, the cell will initially discharge when perisomatic
inhibition is weakest. For theta recorded in the CA1 pyramidal cell
layer, this approximately corresponds with the peak of the oscillation.
On each following cycle as the rat progresses through the field, the cell
will discharge at earlier and earlier phases,[14] typically stopping just
before the trough of the cycle (as recorded in CA1 stratum
pyramidale). In other words, the place cell produces a rhythmic
discharge of a slightly higher frequency than the ongoing theta
oscillation.
Because place fields of different cells overlap, at any particular time
the rat will be at different distances in different fields, so each place
cell will fire at a different phase of theta, allowing the rat's position to
be determined with good precision. This potentially provides an
alternative temporal code for location. Phase precession also results in
the compression of temporal sequences of place cell firing - a
phenomenon believed to facilitate synaptic plasticity.[15] There is
evidence that phase precession is related to depolarisation of the
neuron, such that the firing rate and firing phase of the cell are tightly
coupled,[16] .[17] However, phase precession can also be robustly
independent of firing rate in freely moving animals[18] This caveat of
phase precession, which alludes to the potential neural mechanisms
underlying it, requires further investigation before arriving at a
definitive answer.
Example of phase precession from a rat running
on a circular track. Top plot: The position of the
spikes are plotted along with the phase that the
cell fired relative to the hippocampal theta
rhythm. Bottom plot: Density plot of spike
position versus phase of firing. Note that the
y-axis covers two full theta cycles (0-720
degrees) to ensure that a complete cycle of
precession is seen. The rat enters the field on
right and exits on the left.
References
[1] Skaggs et al., 1996
[2] Moser, E.; Kropff, E.; Moser, M. (2008). "Place cells, grid cells, and the brain's spatial representation system". Annual review of neuroscience
31: 69–89. doi:10.1146/annurev.neuro.31.061307.090723. ISSN 0147-006X. PMID 18284371.
[3] O'Keefe J, Dostrovsky J (1971) "The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat" in
Brain Research Volume 34, pages 171-175. Entrez Pubmed 5124915 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Retrieve&
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[4] John O'Keefe & Lynn Nadel (1978) The Hippocampus as a Cognitive Map (http:/ / www. cognitivemap. net), originally published by Oxford
University Press ISBN 0-19-857206-9).
[5] Ekstrom A, Kahana M, Caplan J, Fields T, Isham E, Newman E, Fried I (2003) "Cellular networks underlying human spatial navigation" in
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[6] Markus E.J., Qin Y.L., Leonard B., Skaggs W.E., McNaughton B.L. and C. A. Barnes (1995) "Interactions between location and task affect
the spatial and directional firing of hippocampal neurons" in Journal of Neuroscience Volume 15, Number 11, pages 7079-7094
[7] Wilson MA, McNaughton BL (1993) "Dynamics of the hippocampal ensemble code for space" in Science Volume 261(5124), pages
1055-1058. Entrez Pubmed 8351520 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Retrieve& db=pubmed& dopt=Abstract&
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[8] Guzowski JF, Knierim JJ, Moser EI (2004) "Ensemble dynamics of hippocampal region CA3 and CA1" in Neuron Volume 44(4), pages
581-584. Entrez Pubmed 15541306 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Retrieve& db=pubmed& dopt=Abstract&
list_uids=15541306)
[9] Knierim JJ, Kudrimoti HS, McNaughton BL (1995) "Place cells, head direction cells, and the learning of landmark stability" in Journal of
Neuroscience Volume 15(3 Pt 1), pages 1648-1659 Entrez Pubmed 7891125 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query.
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[10] Jung MW, Weiner SI, McNaughton BL (1994) "Comparison of spatial firing characteristics of units in dorsal and ventral hippocampus of
the rat" in Journal of Neuroscience Volume 14(12), pages 7347-7356 Entrez Pubmed 7996180 (http:/ / www. ncbi. nlm. nih. gov/ entrez/
query. fcgi?cmd=Retrieve& db=pubmed& dopt=Abstract& list_uids=7996180)
[11] Hargreaves RL, Rao G, Lee I, Knierim JJ (2005) "Major dissociation between medial and lateral entorhinal input to dorsal hippocampus" in
Science Volume 308(5729), pages 1792-1794 Entrez Pubmed 15961670 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query.
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[12] Fyhn M, Molden S, Witter MP, Moser EI, Moser MB (2004) "Spatial representation in the entorhinal cortex" in Science Volume 305(5688),
pages 1258-1264 Entrez Pubmed 15333832 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Retrieve& db=pubmed&
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[13] Hafting T, Fyhn M, Molden S, Moser MB, Moser EI (2005) "Microstructure of a spatial map in the entorhinal cortex" in Nature Volume
436(7052), pages 801-806 Entrez Pubmed 15965463 (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Retrieve& db=pubmed&
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[15] Skaggs WE, McNaughton BL (1996) "Replay of neuronal firing sequences in rat hippocampus during sleep following spatial experience" in
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[16] Harris KD, Henze DA, Hirase H, Leinekugel X, Dragoi G, Czurko A, Buzsaki G (2002) "Spike train dynamics predicts theta-related phase
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External links
• Neural Basis of Spatial Memory (http://www.bris.ac.uk/synaptic/research/projects/memory/spatialmem.
htm), from Bristol University
• Place Cells in the Hippocampus (http://homepages.nyu.edu/~eh597/place.htm)
4
Article Sources and Contributors
Article Sources and Contributors
Place cell Source: http://en.wikipedia.org/w/index.php?oldid=362318503 Contributors: A314268, Bkonrad, Ceyockey, CopperKettle, Diberri, Digfarenough, Fletcher, Gaius Cornelius, Haydes,
Hooperbloob, Icairns, JWSchmidt, Jhuxter, JonathanWilliford, Jpgordon, Looie496, Nrets, O.J.Ahmed, Roadnottaken, Selket, Sgpsaros, Skapur, ThetaMonkey, Torkel, XApple, 32 anonymous
edits
Image Sources, Licenses and Contributors
Image:Triangle-place-cells.png Source: http://en.wikipedia.org/w/index.php?title=File:Triangle-place-cells.png License: Public Domain Contributors: User:Looie496
Image:Phase Precession.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Phase_Precession.jpg License: unknown Contributors: ThetaMonkey
License
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